To produce teeth, tooth parts or dental restorations (hereinafter referred to as tooth), blocks are generally prefabricated and are presintered, for example at a temperature of approximately 900° C. to 1100° C. In this state, the block thus produced can be easily machined, for example by milling. The milling work results in the formation of a shaped article which thereafter simply has to undergo final sintering in order to achieve the required dimension and final hardness. When producing the shaped article, account has to be taken of the shrinkage that occurs in dense sintering. The shrinkage and the strength always derive from the sintering conditions (especially temperature, time, atmosphere) under which the block is produced.
To ensure that the restorations appear as true to nature as possible, it has already been proposed to combine different layers, each differently colored, of ceramic compounds in order to form a tooth. For example, according to this proposal, up to 10 very thin layers of ceramic compounds can be presintered together, such that a shaped article with a suitable color profile can be obtained. If the differently colored layers each have different chemical characteristics, however, there is a danger of the presintering temperature also being different. The compound sintering at higher temperatures then melts on the particle boundaries less strongly than the compound sintering at lower temperatures. This can result in different shrinkages and bond strengths, which make the final production more problematic.
Such block bodies can be made, for example, from ceramics based on ZrO2. However, it is also quite possible to use glasses and also glass ceramics as the starting material. In glass ceramics, primary particles are typically generated from glass granules by comminution, their particle size lying in the range from 0.5 to 50 micrometers. In the context of this invention, particle size and the corresponding indicated sizes are always understood as the d50 value, which corresponds to the diameter at 50% on the frequency distribution curve of particles, as measured according to ISO 13320. Coloring is usually effected by addition of coloring oxides to the melt from which the granulate is obtained, or to the ground granulate. These oxides are then present separately.
To make available an improved block, it has also been proposed to match the color profile to the color profile of an existing tooth. A natural tooth typically has a bright or whitish incisal area and a more strongly colored cervical area. Extending between these areas there is the central area within which the color changes. Between the cervical area and the central area, on the one hand, and between the central area and the incisal area, on the other hand, the color transition weakens, with the result that the variation gradient is typically configured in the manner of a flat Gaussian curve. However, the attempts to simulate a suitable color profile by corresponding layering of ceramic compounds surprisingly led to aesthetically unsatisfactory results, which were also difficult to produce.